Frequency drift indicator



A. KAHN FREQUENCY DRIFT INDICATOR Filed Aug. 28, 1941 May 23, 1944.

Patented May-23, 1944 FREQUENCY narrar INDICATOR Alfred Kahn, Hollis, N. Y., assigner to Radio Corporation of America, a corporation of Dela- Application August 2s, 1941, serial No. 408,633

14 Claims.

This invention relates to frequency drift indi-- cators and has particular reference to a device for checking the frequency of an alternating current with that of a known standard.

In multiplex telegraph systems, it is common practice to maintain synchronsm between local and remote apparatus by meansfof tuning forks and phase correction devices. Usually, the transmitting station is provided with a tuning fork control for the distributor speed. It isalso a common practice to generate an alternating current, the frequency of which is maintained substantially constant by means of a tuning fork. When telegraph signals are transmitted through a multiplex distributor, the speed of which is thus controlled, it is possible, by well-known means, to maintain the speed of a distributor at a receiving station in synchronism with the received signals.

In some of the more extended communications systems, it has been found that a number of different tuning forks are necessary in order to provide all the frequency controls that are necessary. It is desirable to check the frequency of these tuning forks from time to time with an even better standard of frequency. Such a standard is one which is quite costly to build and maintain and, therefore, lprecautions are taken to oper,- ate the standard itself under ideal conditions such as would minimize the frequency drift therein.

`It is an object of this invention to provide convenient means for checking L `rtain practically operable frequency control devices with the frequency of a known standard that may' be re served merely for checking purposes.

It is another object of my invention to provide a convenient arrangement for dividing the fre` titled System for frequency conversion and comparison of frequencies.

In carrying out my invention, I preferably use electronic circuits for deriving the common subharmonic frequency. I then compare the subharmonics obtained from two different sources by meansof a stroboscope. The stroboscope may be.

of the type which embodies a gaseous glow tube rotating on the shaft of a synchronous motor. The speed of the motor may be determined by the frequency of the alternating current derived The 480-cycle frequency is fed across transformer 3 to the input circuit of a discharge tube A 5'. 'I'his tube is self-biased by means of cathode resistor R2; which is shunted by capacitor Cl. The input circuit includes a grid resistor Ri and the secondary of transformer 3.

A limiter 'stage-comprising discharge tube 1 is controlled by output from tube 5. The control potential is developed across resistor R3 the anode-connected end of which is coupled through capacitor C2 and. resistory R! to the control grid of tube 1. Grid leak resistor R4 is connected to the grounded 'cathode of tube 1.

The output from tube 1 is a square wave which serves to synchronize a, multivibrator comprising discharge tubes 9 and Il. 'Ihe circuit constants of these tubes are preferably selected to produce a 40-cycle frequency, thus dividing 'the 480-cycle synchronizing frequency by I2. For tuning the multivibrator the `values of resistors R8, R8, R9, Rill and RII, and of capacitors Q3 and C4 may be determined empirically. Care should be taken that resistors RIB and RII are not subject to ohmic variations due to temperature or humidity. 'I'he values of capacitors C 3 and C4 should also be substantially independent of the temperature and humidity variations, A critical tuning ad- Justinent of the multivibrator is made by fixing the value of resistor-R6 so that the multivibrator will keep in step with the Etwelfth sub-harmonic of the 480-cycle output from tube 1. The anode voltages in tubes 1, 9, and il are adjusted by means of resistor R1.

'I'hegridof tube Il is coupled to the anode of tube 9, andthe grid of tube 9 is coupled to the anode of tube Il. This cross-coupling of the input and output electrodes in the two tubes provides the well known multivibrator action which is desired in order to produce a 40-cycle output `which is synchronized with the 12th sub-harmonic of the 18o-cycle tuning-fork generator.

from the tuning fork. On the other hand, the 56 The square-wave output from tube Il is applied across resistor R|2 and capacitor C5 to the grid of an amplifier tube I3. This tube is self-biased by means of a cathode resistor RII. I'he resonant circuit consisting of inductance I5 and capacitor C5 is connectedbetween 'the grid of tube I3 and ground, thus providing a smoothing action which results in the output from tube Il of a iO-cycle current of essentially sine-wave formation. This current is further smoothed by means of the inductance of an output transformer I1 and' a capacitor C'I, the latter being connected between the anode of tube I 3 and ground.

The secondary of transformer -II is connected to input terminals of a power amplifier I9, which may be of a type commonly used to deliver frefluency-controlled power to one or more synchronous motors. drive telegraph distributors, and the like. In this case, however, the operation of my invention is best explained by showing output `leads from the amplier IS connected to a synchronous motor 2|, the shaft of which carries a small neon lamp 23. The excitation of this lamp is made interdischarge tube 6. This input circuit is tuned by a ycapacitor CI in shunt with the secondary of e transformer l. Resistor RIS also forms part-of the same input circuit.

The first and second discharge zones' of tube l are controlled respectively by grids a and b.

' The output circuits of the first and second zones are cascaded. Grid b is in circuit with resistor RI'I and with the secondary of transformer 8;

The primary of this `transformer is tuned by capacitor. CIU and is fed with an alternating potential developed across resistor RII which is disposed in the output circuit of the first discharge Such motors are, in turn, used tov zone in tube 0. Capacitor C! isolates the direct current anode potential in the first discharge zone of, tube 6 from the grounded primary of transformer I.

Discharge tube I l has input and output circuits which are similar to those of tube 1. Its.` input circuit includes resistors Ris and R20, and the junction between these resistors is coupled across capacitor CII to the anode of the second discharge zone in tube t. A load resistor Ril is provided in the output circuito! this discharge zone. The output circuit of tube il includes resistor R2| across which potentials are developed .suitable for controlling the frequency of a multichosen to produce frequency division by 10 ofv the 10M-cycle synchronizing frequency. The operation of this multivibratorwill, therefore. be understood without further description, except to point out that the=valucs of resistors R25 and of providing such compensation for unbalance as will cause the multivibrator to be maintained in step with an even sub-harmonic (the. tenth) of the 100G-cycle control frequency. Resistor R22 determines the anode potentials applied in tubes I0, I2 and I4.

Output impulses from the multivibrator described in the preceding paragraph are fed through capacitor CI-t and resistor R28 to the input circuit of a succeeding stage comprising a gaseous discharge tube I6. To ignite this'tube, such impulses lare applied negatively to the cathode. The control grid is connected to the cathode through reistors R29 and Rsi, the junction between these two resistors being grounded. Anode potential is suppliedto the tube from a direct current source i8 through resistor R34. In order to adjust the ignition and extinction potentials, resistor R3| is constituted as a variable potentiometer, the movable 'tap on which is connected to one end of resistor R33 for feeding a suitable positive potentialthereto with respect to ground. An appreciable time is required for capacitor CIS to be charged through resistor R34. During this time, thelanode potential in tube i6 builds up gradually to a value such that the next succeeding negative impulse applied to the cathode will produce an ionization discharge. This charge, however, is of only momentary duration since the sudden discharge of capacitor CIE again reduces the anode potential toa value below that `which will sustain ionization. The short sharp discharges in tube I6` result in the delivery of momentary excitation potentials to a neon lamp 23 used for purposes of indicating the frequency drift.

The values of resistor R34 and capacitor CIS are so determined, therefore, that the tube I6 acts asa frequency ldivider and its periodicity is preferably such as to divide the control impulses by 5, thus producing an output of 20 cycles per second, whereas the output fromthe multivibrator tubes' |2 Il was stated to be 100 cycles per second.

One electrode of the lamp 23 is connected through a collector ring and brush and through resistor R30 to the cathodev of tube I6, and the other electrodev of the lamp 23 is grounded.

The lamp 23constitutes an indicating element for a stroboscope, other portions of which include a stationary translucent disc 22 on which there appears a circular scale 'suitably divided for observing the angle of drift of the neon lamp ashes within a given period of time. The lamp 23 is mounted for rotation on the shaft of motor 2|. 'I'he flashes appear through the translucent disc 22. The disc 22 may be graduated in degrees, or

. as an alternative, may be graduated in terms of percentage deviation of the frequency of the tuning fork source with respect to a corresponding ixed standard of the same frequency.

It will be clear from the abovedescription that the flashing of the lamp 23 once per revolution around the scale ofthe disc 22 can be observed as an indication of frequency drift with respect R21 are suitably differentiated forme v75 R.. P. S. and the neon lamp dashing once per degree.

revolution, a convenient means is thus afforded for reading fork generator accuracy directly from thecalibrated dial 22. Furthermore, a short time observation will indicate to the operator whether the working standard is slow or fast and to what 'I'he change in fork speed caused by adjustment can be observed immediately so that amore accurate setting can be made in a brief space of time. The last statement refers to normal operating adjustments made by means of a fork drive potentiometer (not shown) and does not, of course, include major thermostat readjustments which may be found necessary from time to time.

The advantages to be derived from my improved frequency drift' indicator will be better understood from the following discussion of the theory of operation.

By observing'the amount of drift in degrees for a given period of time, the speed difference in parts in 100,000 can be readily determined. For example, in one second. the motor rotation is '20 revolutions, during which time the neon lamp 23 sweeps through '7200 degrees, provided the fork generator frequency is exactly what it should be. The relation between :r degrees of drift per second and the frequency error in parts per 100,000 is shown by the equation- L 1. FZ200-'100,000

or x=.072 per part in 100,000 cycles per second. Thus a shift of .072 per second is equivalent to an accuracy of one part in 100,000. Because of the difficulty of reading so small an angle, it

- is preferable to check for 100 seconds, during which the drift would be 7.2 per part in 100,000.

If R is the observed difference in degrees in 100 seconds, then the frequency drift` may be expressed as 7.2=parts in 100,000

Itis perhaps preferable to divide the circumference of the scale on the disc 22 into 50 divisions.` Each division will then be "1.2, so that the frequency drift in parts in 100,000 will correspond with the number of scale divisions through which the neon flash drifts in 100 seconds.

'Ihe operator by observing the direction of drift of the neon flash can tell whether the 480- cycle fork generator is slow or f'ast. If the drifty is clockwise, this standard is fast; if counterclockwise, it is slow.

Ifhe foregoing description may suggest modifications of my invention, particularly to those skilled in the art, but such modifications should be considered comprehended within the scope of the invention itself.

I claim: f

1. A synchronism indicator for comparing the frequency of a local alternating current source with that of a predetermined standard frequency source, comprising electronic circuits for dividing the frequencies of the two sources by different factors, a motor synchronously driven by an ampliiied current which is a sub-harmonic of the frequency of said local source, a lamp of the gaseous discharge type mounted for rotation on the shaft of said motor, and means for exciting said lamp by short sharp impulses substantially once per revolution of said motor shaft, said impulses being derived from lthe frequency dividing circuit appropriate to said standard frequency source. f

2. An indicator according to claim 1 and including a circular scale mounted coaxially with respectvto the lamps orbit and in a planeparallel thereto. g

3. An indicator according to claim 1 wherein said electronic circuits include multivibrators. each tuned to a subharmonic of the control frequency applied thereto.

4. The method of indicating frequency drift in anv alternating current generator, which com- Aprises deriving a sub-harmonic of the alternating quency dividing means for deriving a sub-liari monic of the output from said generator, a power amplifier under control of the sub-harmonic derivative, a standard frequency source, means for deriving a sub-harmonic of the output from said source the frequency of which is comparable with that of the first said harmonic, a synchronous motor connected for operation by output energy from said power amplifier, a lamp of the gaseous discharge type mounted for rotation on the shaft of said motor, animpulse generator synchronized by a second sub-harmonic derivative from said standard frequency source, and means including slip-ring connections between said impulse generator and said lamp for causing said lamp to produce a stroboscopic indication.

6. Apparatus according to claim 5 wherein-said impulse generator includes a grid-controlled gaseous discharge tube and means including a timeconstant device in the output circuit of said discharge tube for quickly interrupting the discharge therein by reduction of the anode voltage to a value below the discharge extinction level.

41. In a frequency drift indicator for comparing the frequency of an oscillation generator with that of a predetermined standard frequency source, separate amplifier-limiter circuitsconnected one to said generator and the other to said source, said circuits being arranged to deliver substantially square wave outputs at the respective frequencies of said generator and said source, two frequency divider circuits each connected to receive and be controlled by one of said square wave. outputs respectively, a rotatable luminous device, means including a gaseous discharge tube for pulsatively connecting said luminous device to the output side of one of said frequency divider circuits, and means synchronized by the output from the other of said frequency divider circuits for maintaining rotation of said luminous device.

8. The combination according to claim 7 wherein ysaid gaseous discharge tube is arranged to further sub-divide the outputgfrequency of the frequency divider circuit to which it is connected. and to deliver short sharp pulses to said luminous device at a rate commensurate with the revolutions of said device.

9. The combination according to claim 'I wherein said frequency divider circuits are constituted as multivibrators tuned to a desired sub-harmonic of the input frequency applied thereto.

-10. The combination according to claim 'I wherein said gaseous tube is in circuit with a time constant means for periodically building up a discharge potential in the output circuit thereof, said time constant'means being arranged to limit each discharge period to a small fraction ofthe extinction period.

v connected to said generator for independent control thereby, two multivibrator circuits each connected to an appropriate one of said square wave producing means for control thereby, each multivibrator circuit being operative as a frequency divider, a gaseous discharge tube having'an input circuit controlled by output energy from one of said multivibrators, a time constant device for reducing the frequency of discharge pulses in said gaseous tube toa sub-harmonic of the control frequency applied' to its input circuit, a` lamp of the gaseous discharge type mounted for rotation in an indicating orbit, said lamp being connected in shunt with a cathode resistor appropriate to said gaseous discharge tube, and motor meanssynchronously driven byamplied output energy from the other of said multivibrators for causing said lamp to rotate in its orbit at a speed suitable for producing a stroboscopic effect.

13. In a frequency drift indicator, a standard frequency source, an'oscillation generator the normal frequency of which bears a harmonic relation to a submultiple frequency derived from said source, t wo separate wave shaping circuits individually connected toy said source and to said causing the rotation velocity of said gaseous disgenerator respectively for control thereby, said circuits being adapted and arranged to deliver rectangular wave outputs, two multivibrator circuitsl each connected to an appropriate one, of said wave shaping circuits for individual control thereby, each multivibrator circuit being opera` tive as a frequency divider and the normal output frequencies of the two multivibrator circuits being harmonically related, rotatable means for making a luminous indication, said means including a gaseous discharge tube, means for causing said tube to be periodically excited by output pulses from one of said multivibrator circuits, andv means synchronized by output energy from the other of said multivibrator circuits for charge tube to produce a stroboscopic eiect.

14. The combination according to claim 13 and including an electronic limiter stage in each of said wave shaping circuits.

ALFRED KAHN. 

